THE OPPORTUNISTIC SERPENT: MALE GARTER SNAKES ADJUST COURTSHIP TACTICS TO MATING OPPORTUNITIES by R. SHINE 1,2) , T. LANGKILDE 1) and R.T. MASON 3,4) ( 1 School of Biological Sciences A08, University of Sydney, NSW 2006, Australia; 3 Department of Zoology, Oregon State University, Cordley Hall 3029, Corvallis, Oregon 97331-2914, USA) (Acc. 28-VIII-2003) Summary Reproductive males encounter potential mates under a range of circumstances that inuence the costs, benets or feasibility of alternative courtship tactics. Thus, males may be under strong selection to exibly modify their behaviour. Red-sided garter snakes (Thamnophis sir- talis parietalis ) in Manitoba overwinter in communal dens, and court and mate in large aggre- gations in early spring. The number of males within a courting group varies considerably, as do the body sizes of both males and females. We manipulatedthese factors to set up replicated courtship groups in outdoor arenas, and analysed videotapes of 82 courtship trials to quantify courting behaviours of male snakes. Larger and more heavy-bodied males courted more vig- orously than did their smaller, thinner-bodiedrivals, and large females attracted more intense courtship. The major effect, however, involved the number of rival males competing for copu- lation. Males in large groups not only reduced their overall vigour of courtship, but also mod- ied their tactics in such a way as to benet from the courtship activities of rival males. That is, they devoted less energy to inducing female receptivity (which requires energy-expensive caudocephalic waving) and more effort to behaviour (tail-searching)that enhanced their own 2) Corresponding author’s e-mail address: rics@bio.usyd.edu.au 4) We thank Al and Gerry Johnson, Ruth Nesbitt, Heather Waye, Ryan O’Donnell and Deb Lutterschmidt for help and encouragement, and the Manitoba Dept. of Natural Resources for permits. Financial support was provided by the Australian Research Council (to RS), and by a National Science Foundation National Young Investigator Award (IBN-9357245), and the Whitehall Foundation (W95-04) to RTM. Research was conducted under the authority of Oregon State University Institutional Animal Care and Use Committee Protocol No. LAR- 1848B. All research was conducted in accord with the US Public Health Service ‘Policy on Humane Care and Use of Laboratory Animals’ and the National Institutes of Health ‘Guide to the Care and Use of Laboratory Animals’. © Koninklijke Brill NV, Leiden, 2003 Behaviour 140, 1509-1526 Also available online -